References (1,2,3) teach the layout and related fabrication steps for a passive matrix OLED display. However, in passive matrix operation a given pixel emits light only during a line time. In a VGA display, for example, this translates to an optical duty factor of only 1/480=0.21%. To compensate, the pixel must emit 480 times as much light as a pixel that emits constantly. The disadvantages of such operation are (a) higher voltage with attendant higher power, (b) operation at sub-optimum levels of electrical/optical conversion efficiency, (c) possible visual artifacts, and (d) faster degradation of the display. An active matrix OLED display would solve these problems.
Reference (4) teaches a pixel circuit designed for gray scale operation in an active matrix OLED display. This circuit stores the n bits of gray scale in n memory elements at each pixel. However, this circuit requires at least 6n+2 MOS transistors and n column lines, per pixel. Such a circuit would be much too large for use in a practical display. What is needed is a much simpler circuit.
1. U.S. Pat. No. 5,276,380--"Organic EL Image Display Device," C. Tang, Jan. 4, 1994.
2. U.S. Pat. No. 5,294,869--"Organic EL Multicolor Image Display Device," C. Tang and J. Littman, Mar. 15, 1994.
3. U.S. Pat. No. 5,294,870--"Organic EL Multicolor Image Display Device," C. Tang, D. Williams, and J. Chang, Mar. 15, 1994.
4. U.S. Pat. No. 4,996,523--"EL Storage Display with Improved Intensity Driver Circuits," C. S. Bell and M. J. Gaboury, Feb. 26, 1991.
C. W. Tang and S. A. VanSlyke, "Organic EL Diodes," Appl. Phys. Lett. vol. 51, pp. 913-915 (Sep. 21, 1987).
C. W. Tang, S. A. Van Slyke, and C. H. Chen, "Electroluminescence of Doped Organic Thin Films," J. Appl. Phys., vol. 65, pp. 3610-3616 (May 1, 1989).